| Nickel-based superalloys are widely used for aeroengine turbine disks and other high-temperature structural components because of their excellent high temperature mechanical properties,microstructural stability and oxidation resistance.FGH98 is an in-developing third-generation nickel-based powder metallurgy(PM)superalloy for turbine disk of advanced aero-engine in China.The preparation processing of turbine disks made of FGH98 alloy includes argon atomization+hot isostatic pressing+hot extrusion+isothermal forging+ dual-property heat treatment,in which the related fundamental issues such as that of hot deformations and heat treatments,microstructure evolution,property characterization,chemical composition evaluation and optimization are all need to be further studied in-depth.In this paper,the studies mainly focus on the hot deformation behavior of hot extruded FGH98 alloy and its influence on the subsequent heat treatment response,dissolution and precipitation behaviors and laws of ?' precipitates during heating-heat preservation-cooling processes,tensile and creep properties,tensile and creep deformation mechanisms,as well as evaluation and possible direction of optimization of the chemical composition of FGH98 alloy.First,researches on the hot deformation behavior of hot extruded FGH98 alloy and its effect on the subsequent heat treatment response were carried out by thermal simulation technology,the hot deformation temperature,strain rate and true strain adopted were 1060-1165 ?,0.005-10 s-1.and 0.7,respectively.The results show that the deformation temperature 1060-1130? and strain rate of 0.005-0.02 s-1 is the best isothermal forging process parameter set of hot extruded FGH98 alloy,according to which the deforming alloy can undergo superplastic deformation or complete dynamic recrystallization deformation and obtain uniform and fine equiaxed grain structure with average grain size of 1.8-4.3 ?m,and further obtain fine uniform equiaxed grain structure with the size of 4.1-6.2 ?m after subsolvus treatment at 1140? with 20 min,or uniform equiaxed grain structure with the size of 20.3-28.4 ?m after supersolvus treatment at 1180? with 20 min.When deformed at high temperature and low strain rate,the alloy exhibits strain hardening and form inhomogeneous microstructure with irregular grains,which would be inherited in the subsequent heat treatment.During the subsequent supersolvus treatment process,the grains that have completed dynamic recrystallization will grow,while the grains that have not completed dynamic recrystallization will undergo static recrystallization,resulting in inhomogeneous microstructure in the samples that cannot be completely recrystallized dynamically.Interrupted heating and cooling experiments were carried out to study the dissolution and precipitation behaviors of the ?' precipitates in isothermal forged FGH98 alloy in heating(10?/min)and cooling(40?/min)processes.Also,the samples were rapidly heated with a rate of 200?/s to 1109?,1140?,1172?respectively,and holding for different time lengths to study the dissolution behavior of ?' precipitates during the holding process.The results show that the dissolution behavior of the ?' precipitates is affected by the composition and size of ?'precipitates,?/?' interface coherent stress,and matrix dislocation content.The JMAK equation was used to establish the dissolution kinetic equation of ?'precipitates during the holding process.The fitting relationships of the average size and the area fraction of secondary ?' precipitates with temperature were obtained at a cooling rate of 40?/min.The 3D-APT atomic chromatography technique was used to analyze the interface composition distribution of the secondary and tertiary?' phases.It is found that the Al and Ti depleted region appears near the interface of secondary ?' phases.While the tertiary ?' phase interface shows no depleted region of Al and Ti.The experiment results verify that the precipitation of ?' phases during continuous cooling is controlled by the supersaturation caused by cooling and the redistribution of elements between ?/?' phases.The tensile deformation behaviors and mechanisms at room temperature and high temperature(650?,750 ?,815?)of the supersolvus treated FGH98 alloy were studied by SEM and TEM.It is found that as the deformation temperature increases,the stacking fault energy of the alloy decreases,resulting in the dominant tensile deformation mechanism changes from the a/3<112>partial shearing ?'precipitates forming the stacking fault mechanism to the deformation twinning mechanism.A high temperature tensile deformation mechanism in which a/3<112>partial shearing ?' phase forming stacking faults and deformation twins coexist was proposed.The strengthening mechanism of stacking fault shearing and the strengthening and toughening mechanism of deformation twins were explained.The increase of ?' precipitates size promotes the tensile deformation mechanism transition from stacking faults to deformation twins,and improves the plasticity of FGH98 at 750?.The creep properties and creep mechanisms of the supersolvus and subsolvus treated FGH98 alloy were studied by SEM and TEM.It is found that the creep resistance of the coarse-grain structure is better than that of the fine-grain structure at 750?/570 MPa.In the case of low temperature and high stress(650?/870 MPa),Orowan bypass and stacking fault are the dominant creep mechanisms.In the case of medium temperature and medium stress(750?/570 MPa),stacking fault and deformation twinning are the dominant creep mechanisms.In the case of high temperature and low stress(815?/400 MPa),dislocation climbing is the dominate creep mechanism.Finally,the effects of individual alloying elements and specific alloying element combinations on the phase composition and properties of FGH98 alloy were analyzed by Thermo-Calc+TTNi 8 database,and some empirical formulas.On such basis the existing alloy composition of FGH98 alloy was evaluated,and it is suggested that the content of W can be appropriately increased and the content of Mo reduced by keeping the sum of W+Mo unchanged,which would be helpful to further improve the microstructure stability and solid solution strengthening of the alloy. |
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